Repair of ionizing radiation induced damage in mammalian cell DNA is crucial to cell survival. Shoulders on mammalian cell radiation survival curves can be explained by invoking saturation of a repair process at higher doses. To probe this hypothesis, assays of damage are required which can be used at low radiation doses (within the shoulder) as well as high radiation doses. Such assays are proposed here which are effective down to 10 rads, these will be used to test for saturation of repair, measuring rates of repair of DNA single strand breaks, "double strand breaks", and base damage as a function of radiation dose. To probe the significance of these three types of lesions in causing cell death or transformation, attempts will be made to inhibit the repair process and test the result of this inhibition on cell survival or transformation. Also procedures which have been shown to affect cell survival will be probed as to their effects on repair. It is hoped that the information gained from these studies of DNA damage and repair at low doses will be useful in (a) determining the cause of mammalian shouldered survival curves and (b) determining the DNA lesions causing cell death and transformation so that, with the knowledge of cell repair capacity, the hazards of low dose radiation can be extrapolated.